Combined effect of metakaolin and sea water on performance and microstructures of concrete

•The compressive strength of concrete was improved by metakaolin and seawater.•There is a relationship between average pore diameter and compressive strength.•Chloride was immobilized by formation of Friedel’s salt to increase chloride resistance.

The effect of 0–6 wt% MK and mixing with seawater on the properties, hydration and microstructure of concrete was studied. The compressive strength at 28 days increased by 33% when addition of 5 wt% MK and by 22% when mixed with seawater. The combination of both increased compressive strength by 52%. The pore structure was refined under both conditions. There is a relationship between average pore diameter and compressive strength. MK promoted the formation of Friedel’s salt in concrete mixed with seawater. MK and mixing with seawater improved chloride resistance. This study shows that MK and seawater improved the performance of concrete.